Soft demodulation method and apparatus

ABSTRACT

A soft demodulation method and apparatus including calculating partial sums for a unit of each predetermined number of bits of a codeword received from a channel; calculating a value of each entry of the decoding table by referring to the partial sums; and detecting a maximum among values of all entries of the decoding table and calculating a log-likelihood ratio (LLR) using the detected maximum. Accordingly, it is possible to reduce the amount of computation required to perform a soft demodulation process using run-length limited (RLL) codes and to simplify the soft demodulation process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of Korean Patent Application No.2003-5305, filed on Jan. 27, 2003, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to data decoding, and more particularly,to a soft demodulation method and apparatus.

2. Description of the Related Art

In an environment where inter symbol interference (ISI) exists, such asa high-density optical recording medium, data is modulated by usingrun-length limited (RLL) codes.

When conventional RLL codes are decoded, a channel demodulator, such asa Viterbi decoder, detects codewords from a signal input via a channel,and an RLL decoder decodes the codewords into datawords by referring toa decoding table.

Recently, soft demodulators and soft decoders have been increasinglyused for a variety of purposes. The Viterbi decoder directly outputs acodeword, which contains an error and simply has a value of “1” or “−1”.This type of modulation is called hard demodulation. In softdemodulation, however, a soft demodulator receives a channel signal andoutputs a probability value of a codeword. In other words, the output ofthe soft demodulator has an analog value, such as “0.8” or “−0.8”,containing the probability of a codeword having a value of 1 or −1. Thesoft demodulator receives data indicating the probability value of thecodeword and outputs a probability value of a dataword. Then, a softencoder, such as a turbo encoder, receives the probability of thedataword from the soft demodulator and encodes the codeword into thedataword.

A turbo decoding process that requires soft demodulation is taught byLaura L. McPheters and Steven W. McLaughlin in “Turbo-Coded OpticalRecording Channels with DVD Minimum Mark Size,” IEEE Transactions onMagnetics, Vol. 38, No. 1, January 2002, pp. 298˜302.

FIG. 1 is a block diagram of a conventional encoding/decoding apparatus.Referring to FIG. 1, an encoding/decoding apparatus 100 includes aturbo/low density parity check code (LDPC) encoder 110, a modulator 120,a reader/writer 130, a soft demodulator 150, and a turbo/LDPC decoder160.

Data is input to the turbo/LDPC encoder 110, which encodes the data byusing a soft encoding method, such as an LDPC encoding method or a turboencoding method. The modulator 120 modulates data output from theturbo/LDPC encoder 110 in a predetermined demodulation manner that uses,for example, an RLL code. The reader/writer 130 records the modulateddata on a recording medium 140 or reads data from the recording medium140. The soft demodulator 150 receives data indicating a probabilityvalue of a codeword from the reader/writer 130 and then outputs a loglikelihood ratio (LLR), which corresponds to a probability value of eachbit of a dataword. The turbo/LDPC decoder 160 carries out a softdecoding on data output from the soft demodulator 150 taking intoconsideration how the corresponding data has been encoded. Thereafter,the turbo/LDPC decoder 160 outputs the soft-decoded data.

The soft demodulator 150 obtains the LLR by calculating a posterioriprobability (APP) (d_(k)=1) and APP(d_(k)=0). APP(d_(k)=1) representsthe probability of demodulated data having a value of “1”, andAPP(d_(k)=0) represents the probability of demodulated data having avalue of “0”. APP(d_(k)=1) and APP(d_(k)=0) are obtained in thefollowing manner. If a codeword, which is used for determining adataword, is comprised of N bits, values of (r_(m)−2*c_(m)−1))² (wherem=1, . . . , N) for bits of each codeword that sets a bit of thedataword to 1 are summed up. In short, APP(d_(k)=1) can be obtained bysumming up an exponential value of (r_(m)−2*c_(m)−1))² for each codewordthat sets a bit of the dataword to 1, which is shown in Equation (1)below.

$\begin{matrix}{{{APP}\left( {d_{k} = 1} \right)} = {\sum\limits_{j \in {S_{1}{(k)}}}{\exp\left\lbrack {r_{m}^{j} - \left( {{2*c_{m}^{j}} - 1} \right)^{2}} \right\rbrack}}} & (1)\end{matrix}$

In Equation (1), j indicates that a j-th dataword has a value of 1.

Likewise, APP(d_(k)=0) is obtained using Equation (2) below.

$\begin{matrix}{{{APP}\left( {d_{k} = 0} \right)} = {\sum\limits_{j \in {S_{0}{(k)}}}{\exp\left\lbrack {r_{m}^{j} - \left( {{2*c_{m}^{j}} - 1} \right)^{2}} \right\rbrack}}} & (2)\end{matrix}$

LLR(d_(k)) is obtained by exponentiating a ratio between the probabilityAPP(d_(k)=0) of the j-th bit of the dataword d_(k) having a value of 0and the probability APP(d_(k)=1) of the j-th bit of the dataword d_(k)having a value of 1, a process which is shown in Equation (3) below.LLR(d_(k)) is an output of the soft demodulator 150.

$\begin{matrix}{{{LLR}\left( d_{k} \right)} = {{Log}\frac{\left( {{{APP}\left( d_{k} \right)} = 1} \right)}{\left( {{{APP}\left( d_{k} \right)} = 0} \right)}}} & (3)\end{matrix}$

If it is too complicated to exponentiate the ratio between APP(d_(k)=1)and APP(d_(k)=0), LLR(d_(k)) can be obtained by subtracting APP(d_(k)=0)from APP(d_(k)=1). The structure of the soft demodulator 150, whichcalculates LLR(d_(k)) in this manner, is illustrated in FIG. 2.

Referring to FIG. 2, the soft demodulator 150 includes a decoding table151, an entry calculator 152, a maximum detector 153, and an LLRcalculator 154. The decoding table 151 is illustrated in FIG. 3.

The entry calculator 152 performs a predetermined calculation on a readcode signal using each entry of the decoding table 151. Hereinafter, thepredetermined calculation will be described more fully with reference toFIG. 4. More specifically, the entry calculator 152 associates each bitof an input code signal 410 with each bit of the entry 420. If a bit ofthe entry 420, corresponding to a given bit of the input code signal410, has a value of 0, the given bit of the input code signal 410 ismultiplied by −1. On the other hand, if a bit of the entry 420,corresponding to the given bit of the input code signal 410, has a valueof 1, the given bit of the input code signal 410 is multiplied by +1.The entry calculator 152 performs this type of multiplication on eachbit of the input code signal 410 and then sums up all multiplicationresults, thus obtaining a result 430 of the predetermined calculation.

For example, assume that the input code signal 410 is “r₀r₁ . . . r₁₇”and the entry is “000101 . . . 100”, as shown in FIG. 4. Because thefourth, sixth, ninth, eleventh, fourteenth, and sixteenth bits of theentry 420 have a value of 1 and the other bits of the entry 420 have avalue of 0, r₃, r₅, r₈, r₁₀, r₁₃, and r₁₅ are multiplied by +1, and theother bits of the are multiplied by −1. Thereafter, the multiplicationresults are summed up. The entry calculator 152 carries out thepredetermined calculation on the input code signal 410 as many times asthe number of entries of the decoding table 151.

The maximum detector 153 receives as many results of the predeterminedcalculation from the entry calculator 152 as the number of entries ofthe decoding table 151 and detects a maximum among the predeterminedcalculation results.

The LLR calculator 154 calculates LLR(d_(k)) by subtracting a maximumregarding “0” from a maximum regarding “1”, by a process as shown inEquation (4) below. Thereafter, the LLR calculator 154 outputsLLR(d_(k)).

$\begin{matrix}\begin{matrix}{{{LLR}\left( d_{k} \right)} = {{\max_{j \in {S_{1}{(k)}}}\left\lbrack {\sum\limits_{m = 0}^{t - 1}{r_{m}^{j}*2\left( {c_{m}^{j} - 1} \right)}} \right\rbrack} -}} \\{\max_{j \in {S_{0}{(k)}}}\left\lbrack {\sum\limits_{m = 0}^{t - 1}{r_{m}^{j}*2\left( {c_{m}^{j} - 1} \right)}} \right\rbrack}\end{matrix} & (4)\end{matrix}$

LLR(d_(k+1)) can be obtained by simply substituting k+1 into Equation(4), as shown in Equation (5) below.

$\begin{matrix}\begin{matrix}{{{LLR}\left( d_{k + 1} \right)} = {{\max_{j \in {S_{1}{({k + 1})}}}\left\lbrack {\sum\limits_{m = 0}^{t - 1}{r_{m}^{j}*2\left( {c_{m}^{j} - 1} \right)}} \right\rbrack} -}} \\{\max_{j \in {S_{0}{({k + 1})}}}\left\lbrack {\sum\limits_{m = 0}^{t - 1}{r_{m}^{j}*2\left( {c_{m}^{j} - 1} \right)}} \right\rbrack}\end{matrix} & (5)\end{matrix}$

In Equation (5), S₀(k+1) indicates a set of entries of the decodingtable 151 of FIG. 3, and S₁(k+1) indicates a set of entries of adecoding table, which is very similar to the decoding table 151.

If soft modulation is carried out using the decoding table 151, as manyas 85,000 calculations are required. Here, 85,000 is obtained bymultiplying a total number of additions and subtractions using bits ofeach entry of the decoding table 151, i.e., 17, by the number of entriesof the decoding table 151, i.e., 5,000.

As described above, in the related art, as the size of an APP decodingtable used for soft demodulation increases, the time taken to calculatean LLR increases, and the APP decoding table becomes remarkablycomplicated.

SUMMARY OF THE INVENTION

The present invention provides a soft demodulation method and apparatus,which can reduce the complexity of a decoding table used in softdemodulation and the amount of computation required to carry out thesoft demodulation.

According to an aspect of the present invention, there is provided asoft demodulation method. The soft demodulation method involvescalculating partial sums for a unit of each predetermined number of bitsof a codeword received from a channel; calculating a value of each entryof the decoding table by referring to the partial sums; and detecting amaximum among values of all entries of the decoding table andcalculating a log-likelihood ratio (LLR) using the detected maximum.

Additional aspects and/or advantages of the invention will be set forthin part in the description which follows and, in part, will be obviousfrom the description, or may be learned by practice of the invention.

In an aspect of the present invention, the partial sums for the unit ofeach predetermined number of bits of the codeword is calculated by usingreference entries, and each of the reference entries is comprised of acombination of some bits of each of the entries of the decoding table.

In an aspect of the present invention, the calculating the value of eachentry includes dividing each of the entries of the decoding table intobit units, each of the bit units comprised of the predetermined numberof bits; and calculating the value of each of the entries of thedecoding table by summing up the partial sums corresponding to the bitunits of each of the entries of the decoding table.

In an aspect of the present invention, if partial sums corresponding tobit units of a predetermined entry of the decoding table do not exceed apredetermined threshold value, a value of the predetermined entry is notcalculated.

According to another aspect of the present invention, there is provideda soft demodulation apparatus. The soft demodulation apparatus includesa partial sum calculator, which calculates partial sums for a unit ofeach predetermined number of bits of a codeword received from a channel;an entry calculator, which calculates a value of each entry of thedecoding table by referring to the partial sums; a maximum detector,which detects a maximum among values of all entries of the decodingtable; and a log-likelihood ratio (LLR) calculator, which calculates anLLR using the detected maximum.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the invention will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 is a schematic block diagram of a conventional dataencoding/decoding apparatus;

FIG. 2 is a block diagram of the soft demodulator of FIG. 1;

FIG. 3 is a diagram illustrating an example of a decoding table of FIG.2;

FIG. 4 is a diagram illustrating the operation of an entry calculator ofFIG. 2;

FIG. 5 is a flowchart of a soft demodulation method according to anembodiment of the present invention;

FIG. 6 is a block diagram of a soft demodulation apparatus according toan embodiment of the present invention;

FIG. 7 is a diagram illustrating an example of a reference entry of FIG.6;

FIG. 8 is a diagram illustrating an example of a partial sum table ofFIG. 6;

FIG. 9 is a diagram illustrating another example of the partial sumtable of FIG. 6;

FIG. 10 is a diagram illustrating still another example of the partialsum table of FIG. 6;

FIG. 11 is a flowchart of a soft demodulation method according to anembodiment of the present invention; and

FIG. 12 is a block diagram of a soft demodulation apparatus according toan embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to the like elementsthroughout. The embodiments are described below to explain the presentinvention by referring to the figures.

FIG. 5 is a flowchart of a soft demodulation method according to anembodiment of the present invention. Referring to FIG. 5, a code signal,which is comprised of 18 bits, is received from a channel in operationS510. In operation S520, partial sums for bits of the received codesignal are calculated using reference entries. Here, the referenceentries are unit entries each comprised of a predetermined number ofbits selected from bits constituting each entry of a decoding table.Each entry of the decoding table, like the decoding table 151 of FIG. 3,may be comprised of 18 bits. However, the decoding table does not haveas many entries as the total number of combinations of the 18 bits,i.e., 2¹⁸. Each entry is comprised of a combination of 5 unit entries,i.e., 5 reference entries. Each of the 5 reference entries comprises 3bits. The five reference entries are “000”, “001”, “010”, “100”, and“101”.

In operation S520, partial sums of each three bits of the received codesignal are calculated by referring to the reference entries. Forexample, partial sums of the first three bits r₀, r₁, and r₂ of thereceived code signal are calculated by using each of the referenceentries “000”, “001”, “010”, “100”, and “101”. As a result, partial sums−r₀−r₁−r₂, −r₀−r₁+r₂, −r₀+r₁−r₂, +r₀−r₁−r₂, and +r₀−r₁+r₂ are obtainedof the first three bits r₀, r₁, and r₂ of the received code signal.Thereafter, partial sums −r₃−r₄−r₅, −r₃−r₄+r₅, −r₃+r₄−r₅, +r₃−r₄−r₅, and+r₃−r₄+r₅ are obtained of the next three bits r₃, r₄, and r₅ of thereceived code signal. In this manner, partial sums for the rest of thereceived code signal are calculated by using the five reference entries.

In operation S530, each entry value is calculated using the five partialsums for each three bits of the received code signal. In other words, ifan entry is “000 101 001 010 010 100”, a partial sum entry correspondingto each three bits of the entry is searched and then is summed up. Morespecifically, the first three bits of the entry, i.e., “000”, correspondto the partial sum entry −r₀−r₁−r₂, and the next three bits of theentry, i.e., “101”, correspond to the partial sum entry +r₃−r₄+r₅. Inthis manner, the value of the entry is obtained by searching for andsumming up the partial sum entry corresponding to each three bits of theentry. Operation S530 is performed on each of the 5000 entries of thedecoding table, thus obtaining 5000 calculated entry values.

In operation S540, a maximum among the 5000 calculated entry values isdetected. In operation S550, an LLR is calculated using the detectedmaximum and then is output.

When the decoding table has, for example, 5000 entries and there are 6partial sum tables provided, a total number of calculations requiredamounts to 25,060 (=5 (the number of partial sum entries)×6 (the numberof partial sum tables)×2 (the number of calculations performed on eachentry)+5000 (the number of entries of the decoding table)×5 (the numberof partial sums to be summed up for the calculation of one entryvalue)).

FIG. 6 illustrates a soft demodulation apparatus 600 according to anembodiment of the present invention, which performs the softdemodulation method of FIG. 5. Referring to FIG. 6, the softdemodulation apparatus 600 includes a reference entry unit 610, apartial sum calculator 620, a partial sum table 630, an entry calculator640, a maximum detector 650, an LLR calculator 660, and a decoding table670.

FIG. 7 shows an example of the reference entry unit 610, which includesfive reference entries “000” (611), “001” (612), “010” (613), “100”(614), and “101” (615).

The partial sum calculator 620 receives the reference entries 611through 615 from the reference entry unit 610 and each three bits of acode signal read from a channel and calculates partial sums for eachthree bits of the code signal using the received reference entries. Forexample, supposing that three bits r₀, r₁, and r₂ of the code signal arereceived, partial sums −r₀−r₁−r₂, −r₀−r₁+r₂, −r₀+r₁−r₂, +r₀−r₁−r₂, and+r₀−r₁+r₂ are obtained by applying the reference entries “000”, “001”,“010”, “100”, and “101”, respectively, to the three bits r₀, r₁, and r₂of the code signal.

When the calculation of partial sums for each three bits of the codesignal using the reference entries is completed, the partial sum table630 is formed. An example of the partial sum table 630 is illustrated inFIG. 8.

Referring to FIG. 8, the partial sum table 800 includes 6 sub-tables810, 820, 830, 840, 850, and 860, and each of the sub-tables 810, 820,830, 840, 850, and 860 includes five partial sums.

The entry calculator 640 calculates values of entries of the decodingtable 670 by using the partial sums of the partial sum table 630. Forexample, if a first entry of the decoding table 670 is “000 101 001 010010 100”, partial sums corresponding to each three bits of the firstentry are searched for in the partial sum table 630 and are addedtogether.

For example, the first three bits “000” of the first entry correspond tothe partial sum entry 811, i.e., −r₀−r₁−r₂, and the next three bits“101” of the first entry correspond to the partial sum entry 825, i.e.,+r₃r−r₄+r₅. In this manner, partial sums for the rest of the first entryare searched for in the partial sum table 800. The partial sums for eachthree bits of the example first entry are shown in the following table.

TABLE 1 Bits of Entry of Decoding Table Partial sums 000 −r₀−r₁−r₂ 101+r₃−r₄+r₅ 001 −r₆−r₇+r₈ 010 −r₉+r₁₀−r₁₁ 010 −r₁₂+r₁₃−r₁₄ 100+r₁₅−r₁₆−r₁₇

A result of summing up the partial sums for each three bits of the firstentry is a value of the first entry. The entry calculator 640 calculatesthe values of each of the 5000 entries of the above-described manner.

The maximum detector 650 detects a maximum among the 5000 entry valuesobtained by the entry calculator 640.

The LLR calculator 660 calculates an LLR using the maximum detected bythe maximum detector 650.

FIG. 9 illustrates another example of the partial sum table 630.Referring to FIG. 9, the partial sum table 900 includes a sub-table 910,into which the sub-tables 810 and 820 of the partial sum table 800illustrated in FIG. 8 are integrated, a sub-table 920, into which thesub-tables 830 and 840 are integrated, and a sub-table 930, into whichthe sub-tables 850 and 860 are integrated. Each of the sub-tables 910,920, and 930 includes 25 (5×5) entries.

In the case of calculating the entry values of the decoding table 670using the partial sum table 900, the total number of calculationsrequired amounts to 10,135 (=30×2 (the number of calculations requiredto form the six sub-tables 810 through 860 of the partial sum table800)+3×25 (the number of calculations required to form the threesub-tables 910 through 930 of the partial sum table 900)+5000×2 (thenumber of calculations required to calculate the 5000 entry values ofthe decoding table 670)).

FIG. 10 illustrates still another example of the partial sum table 630.Referring to FIG. 10, the partial sum table 1000 includes a sub table1010, into which the sub-tables 810, 820 and 830 of the partial sumtable 800 are integrated, and a sub table 1020, into which thesub-tables 840, 850, and 860 are integrated. Therefore, each of thesub-tables 1010 and 1020 includes 125(5×5×5) entries.

In the case of calculating the entry values of the decoding table 670using the partial sum table 1000, the total number of calculationsrequired amounts to 5,310 (=30×2 (the number of calculations required toform the six sub-tables 810 through 860 of the partial sum table800)+2×125 (the number of calculations required to form the twosub-tables 1010 and 1020 of the partial sum table 1000)+5000×1 (thenumber of calculations required to calculate the 5000 entry values ofthe decoding table 670)).

FIG. 11 is a flowchart of a soft demodulation method according to anembodiment of the present invention. If the partial sums of each entryof the decoding table 670 do not reach a predetermined value, i.e., athreshold value, the partial sums are ignored rather than being summedup to calculate an entry value of the decoding table 670. Therefore, itis possible to reduce the amount of computation required.

For example, assume that the threshold value is set to 0, the firstentry of the decoding table 670 is “000 101 001 010 010 100”, and thepartial sum table 630 of FIG. 6, like the partial sum table 1000 of FIG.10, is divided into two sub tables 1010 and 1020. A partial sum entryr₀+r₁+r₂+r₃+r₄+r₅+r₆+r₇+r₈ is searched for in the first sub-table 1010for first nine bits of the first entry, and a partial sum entryr₉+r₁₀+r₁₁+r₁₂+r₁₃+r₁₄+r₁₅+r₁₆+r₁₇ is searched for in the secondsub-table 1020 for the rest of the first entry. If at least one of thepartial sums is smaller than the threshold value, i.e., 0, the partialsums are ignored rather than to be summed up to calculate the value ofthe first entry, and then a process of calculating a value of a secondentry of the decoding table 670 is carried out. In other words, if apartial sum necessary for calculating a predetermined entry value has avalue smaller than the threshold value, the predetermined entry value isnot calculated. The threshold value may be set to a different value than0 according to a channel state. If no partial sum entry exceeds thethreshold value, the calculation of each of the entry values of thedecoding table 670 should be performed again with the threshold valueset lower.

Referring to FIG. 11, a code signal is received from a channel inoperation S1110. In operation S1120, partial sums are calculated usingreference entries. Operation 1120 is the same as operation S520 of FIG.5. In operation S1130, it is determined whether each of the partial sumscalculated in operation S1120 exceeds a predetermined threshold value.In operation S1140, if each of the partial sums does not exceed thepredetermined threshold value, the partial sums are ignored. Otherwise,an entry value of a decoding table is calculated by summing up thepartial sums. In operation S1150, a maximum among all entry values ofthe decoding table is detected. In operation S1160, an LLR is calculatedusing the maximum detected in operation S1150 and then is output.

If the partial sums of each entry of the decoding table do not reach apredetermined value, i.e., a threshold value, the partial sums areignored rather than being summed up to calculate an entry value of thedecoding table. Therefore, the amount of computation required tocalculate entry values may be reduced.

FIG. 12 is a block diagram of a soft demodulation apparatus 1200according to an embodiment of the present invention, which performs thesoft demodulation method of FIG. 11. Referring to FIG. 12, the softdemodulation apparatus 1200 includes a reference entry unit 1210, apartial sum calculator 1220, a partial sum table 1230, a threshold valuedeterminator 1240, an entry calculator 1250, a maximum detector 1260, anLLR calculator 1270, and a decoding table 1280.

The soft demodulation apparatus 1200 of FIG. 12 is the same as the softdemodulation apparatus of FIG. 6 except that it includes the thresholdvalue determinator 1240.

In other words, the reference entry unit 1210, like the reference entryunit 610 illustrated in detail in FIG. 7, includes reference entries“000”, “001”, “010”, “100”, and “101”. The partial sum calculator 1220calculates partial sums by referring to the reference entries of thereference entry unit 1210. The partial sum table 1230 stores the partialsums calculated by the partial sum calculator 1220.

The entry calculator 1250 searches the partial sum table 1230 forpartial sums corresponding to a predetermined entry. Then, the thresholdvalue determinator 1240 determines whether the partial sums searched forby the entry calculator 1250 exceed a predetermined threshold value.

If the partial sums corresponding to the predetermined entry exceed thepredetermined threshold value, the entry calculator 1250 calculates avalue of the predetermined entry by summing up the partial sums.Otherwise, the partial sums are abandoned, and then a value of a nextentry is calculated without calculating the value of the predeterminedentry.

Thus, to save computation resources a partial sum entry that is lesslikely to be detected as a maximum is abandoned, and the value of acorresponding entry is not calculated.

According to the present invention, it is possible to reduce the amountof computation required to perform a soft demodulation process usingrun-length limited (RLL) codes and to simplify the soft demodulationprocess.

Although a few embodiments of the present invention have been shown anddescribed, it would be appreciated by those skilled in the art thatchanges may be made in this embodiment without departing from theprinciples and spirit of the invention, the scope of which is defined inthe claims and their equivalents.

1. A soft demodulation method, comprising: calculating partial sums for a unit of each predetermined number of bits of a codeword received from a channel; calculating a value of each entry of a decoding table by referring to the partial sums; and detecting a maximum among values of all entries of the decoding table and calculating a log-likelihood ratio (LLR) using the detected maximum, wherein the partial sums for the unit of each predetermined number of bits of the codeword is calculated by using reference entries, and each of the reference entries is comprised of a combination of some bits of each of the entries of the decoding table.
 2. The soft demodulation method of claim 1, wherein the calculating the value of each entry comprises: dividing each of the entries of the decoding table into bit units, each of the bit units comprising the predetermined number of bits; and calculating the value of each of the entries of the decoding table by summing up the partial sums corresponding to the bit units of each of the entries of the decoding table.
 3. The soft demodulation method of claim 2, wherein each of the bit units is comprised of 3 bits, 6 bits, or 9 bits.
 4. The soft demodulation method of claim 2, wherein, if the partial sums corresponding to the bit units of a predetermined entry of the decoding table do not exceed a predetermined threshold value, a value of the predetermined entry is not calculated.
 5. The soft demodulation method of claim 1, wherein the reference entries are “000”, “001”, “010”, “100”, and “101”.
 6. A soft demodulation apparatus, comprising: a partial sum calculator, which calculates partial sums for a unit of each predetermined number of bits of a codeword received from a channel; an entry calculator, which calculates a value of each entry of a decoding table by referring to the partial sums; a maximum detector, which detects a maximum among values of all calculated entries of the decoding table; and a log-likelihood ratio (LLR) calculator, which calculates an LLR using the detected maximum, wherein the partial sum calculator calculates the partial sums of each predetermined number of bits of the codeword by using reference entries, and each of the reference entries comprises a combination of some bits of each of the entries of the decoding table.
 7. The soft demodulation apparatus of claim 6, wherein the entry calculator divides each of the entries of the decoding table into bit units each comprising the predetermined number of bits and sums up the partial sums corresponding to the bit units of each of the entries of the decoding table.
 8. The soft demodulation apparatus of claim 6 further comprising: a threshold value determinator, which determines whether the partial sums corresponding to the bit units of each of the entries of the decoding table exceed a predetermined threshold value, wherein if the partial sums corresponding to bit units of a predetermined entry of the decoding table do not exceed a predetermined threshold value, the entry calculator does not calculate a value of the predetermined entry.
 9. An optical signal processing apparatus of an optical disc device, comprising: a read/write unit to output a read code signal from an optical disc in the optical disc device; a reference entry unit which stores a predetermined number of reference entries; a partial sum calculator which calculates partial sums of n-bit portions of the read code signal received from the read/write unit corresponding to each stored reference entry; a partial sum table which stores the calculated partial sums corresponding to each n-bit portion of the read code signal; an entry calculator which calculates an entry value of each entry of a decoding table by summing the corresponding calculated partial sums stored in the partial sum table that correspond to an n-bit portion of the decoding table entry; a maximum detector which detects a maximum calculated entry value; and a log-likelihood ratio (LLR) calculator, which calculates an LLR using the detected maximum.
 10. The apparatus of claim 9, wherein the partial sum table comprises sub tables corresponding to each n-bit portion of the read code signal.
 11. The apparatus of claim 9, wherein the partial sum table comprises sub tables corresponding to each 2n-bit portion of the read code signal.
 12. The apparatus of claim 9, wherein the partial sum table comprises sub tables corresponding to each 3n-bit portion of the read code signal.
 13. The apparatus of claim 9, further comprising a threshold determiner to control the entry calculator to calculate the decoding table entry based on the calculated partial sums stored in the partial sum table exceeding a predetermined threshold, wherein when the calculated partial sums are below the predetermined threshold the entry calculator does not calculate the decoding table entry.
 14. A soft demodulation method, comprising: storing predetermined reference entries; calculating partial sums corresponding to n-bit portions of a read code signal based on the stored predetermined reference entries; storing the calculated partial sums; calculating an entry value of each entry of a decoding table by adding the stored calculated partial sums which correspond to consecutive n-bit portions of the entry of the decoding table; detecting a maximum entry value; and calculating a log-likelihood ratio based on the detected maximum entry value.
 15. The method of claim 14, wherein the storing the calculated partial sums comprises storing sub tables of the calculated partial sums corresponding to each n-bit portion of the read code signal.
 16. The method of claim 14, further comprising determining if the calculated partial sums exceed a predetermined threshold, wherein when the calculated partial sums exceed the predetermined threshold the entry value of the decoding table is calculated and when the calculated partial sums are below the predetermined threshold the entry value of the decoding table is not calculated.
 17. A soft demodulation method, comprising: calculating partial sums corresponding to a codeword; calculating a predetermined entry value of a decoding table, by adding the partial sums, wherein the partial sums are abandoned when the partial sums are less likely to be detected as a maximum and the partial sums are added together to calculate the predetermined entry value when the partial sums are more likely to be detected as a maximum; and detecting a maximum among the calculated predetermined entry values of the decoding table and calculating a log-likelihood ratio (LLR) using the detected maximum.
 18. A soft demodulation method, comprising: calculating an entry value of a decoding table by calculating partial sums of predetermined portions of a codeword, wherein the entry value is skipped when the partial sums are below a predetermined threshold; and detecting a maximum of the calculated entry values of the decoding table and calculating a log-likelihood ratio (LLR) using the detected maximum. 